The last four decades have witnessed profound changes in American family composition. The drastic decline in marriage rates has coincided with a steep increase in the non-marital birth rate. Family composition shapes social, behavioral, and economic characteristics. Children raised by single parents are more likely to display delinquent behavior. Daughters raised by single mothers are more likely to engage in early sexual activity; their brothers are twice as likely to spend time in jail. They are less likely to finish high school. They are four to five times as likely to live in poverty. If we want to address the challenges of income inequality and immobility, we must address one of their main causes—non-marital births and single parenting. It is essential that analysts and policymakers come to terms with what our experience can teach us, so they can seek to build on what works.